The main effect of γ-ray irradiation on KNbO 3 single crystals is to produce impurity dipoles in the structure. An oxygen vacancy is created on the oxygen octahedron, and the impurity ion within the octahedron forms a dipole with the electrons associated with the vacancy. Some impurity dipoles of this type are already present in the structure. In irradiated crystals heated to 500 °C, well above the Curie temperature, annealed there for 12 h to achieve homogenization as far as possible, and then slow-cooled at a rate of 20 ° h −1, it was found that domain nucleation by the impurity dipoles at the Curie temperature depends entirely on the dipole density attained as a result of irradiation. At low densities below ~10 9 cm −3, order amongst the dipoles is absent. This order exists in a narrow dipole density range; and only when the dipoles are ordered can they nucleate domains around them. This information can be used to control domain structures in the crystal, particularly when it is the main mechanism of domain formation. It is suggested that the ordering of the dipoles and domain nucleation by them in a limited range of dipole density can be explained in terms of the elastic effects associated with the impurity dipoles in the crystal structure.